Anode Layer Sources (ALSs) produce and accelerate ions from a thin and intense plasma called the “anode layer”. This anode layer forms adjacent to an anode surface of an ALS due to large Hall currents, which are generated by the interaction of strong crossed electric and magnetic fields in the plasma discharge (gap) region. This plasma discharge region is defined by the magnetic field gap between cathode pole pieces (also called the “cathode-cathode gap”) and the electric field gap between the downstream surface of the anode and the upstream surface of the cathode (also called the “anode-cathode gap”). A working gas, including without limitation a noble gas, oxygen, or nitrogen, is injected into the plasma discharge region and ionized to form the plasma. The electric field accelerates the ions away from the plasma discharge region toward a substrate.
In one implementation of a linear ALS, the anode layer forms a continuous, closed path exposed along a race-track-shaped ionization channel in the face of the ion source. Ions from the plasma are accelerated primarily in a direction normal to the anode surface, such that they form an ion beam directed roughly perpendicular to the ionization channel and the face of the ion source. Different ionization channel shapes may also be employed.
For typical etching or surface modification processes, a substrate (such as a sheet of flat glass) is translated through the ion beam in a direction perpendicular to the longer, straight sections of the ionization channel. Uniform etching across the substrate, therefore, depends on the ion beam flux and energy density being uniform along the length of these straight channel sections. Variations in the ion beam flux and energy density uniformity along the straight channel sections can significantly degrade the longitudinal uniformity of the resulting ion beam.
Non-uniformities in the anode-cathode gap can have a significant negative effect on the longitudinal ion beam uniformity and can be introduced in various ways during manufacturing. For example, the ion source body can be warped by the welding or brazing of a cooling tube to the outside surface of the ion source body, thus introducing anode-cathode gap variations.
Minor gap variations can result in substantial longitudinal beam current density variations. A typical ALS geometry has an anode-cathode gap of 2 mm, a cathode-cathode gap of 2 mm, and a cathode face height of 2 mm, which is also known as a 2×2×2 mm geometry. Measurements of a linear ALS using this geometry have shown that variations of 0.3 mm in the anode-cathode gap dimension can cause longitudinal beam current density variations of 8%. It should be understood that alternative ALS configurations and dimensions may also be employed. Non-uniformities in the cathode-cathode gap and the working gas distribution to the anode layer can also negatively influence ion beam uniformity.
A typical ALS design includes a rigid monolithic anode supported on insulators in a cavity of a rigid monolithic source body. Both the anode and the source body are cut from stainless steel stock and are precisely machined to the desired dimensions. Rough machining and welding-induced or brazing-induced distortion during assembly often dictate that the flat surfaces of the source body and anode undergo a final precision machining operation in order to hold the desired gap dimension tolerance.
This manufacturing process has provided good results for relatively short ion sources (e.g., 300 mm long). However, some ALS applications can require very long ion sources (e.g., 2540 mm to 3210 mm). For example, some architectural glass processing applications can require an ALS that is about twelve feet long (i.e., 3657.6 mm). Such length can make it extremely difficult and prohibitively expensive to maintain the required uniformity of the anode-cathode gap over the entire length of the ALS. Therefore, using traditional monolithic designs and manufacturing techniques for long ALSs is undesirable and potentially infeasible.
Differential thermal expansion of a cathode plate during operation is a particular problem with long linear ion sources. The inner edge of the cathode plate is directly exposed to a very intense plasma discharge and operates at a very high temperature, whereas the outer edge or region of the cathode plate, which is in direct contact with a water-cooled surface, operates at a significantly lower temperature. The temperature difference between the inner edge and the outer edge/region of a cathode plate can introduce a variety of problems in ion source operation, including non-uniformities in the ion beam and damage to the cathode and the attachment bolts that secure the cathode to the source body.
In addition, cathodes are traditionally precision machined out of thick stainless steel stock, which makes a cathode an expensive component. To compound this expense, operation of the ion source results in substantial wearing of the inner edge of each cathode plate, which can degrade the uniformity of the cathode face height, the cathode-cathode gap, the anode-cathode gap, and the electric and magnetic fields in the gap. Accordingly, such cathode wear necessitates frequent replacement of cathode plates during the life of the ion source.